Hello,
this is a real log of debugging GCC (with stage1 C miscompiling stage2
Fortran), including finding the miscompilation and fixing it. I hope it
helps understanding the way toplevel bootstrap works.
../configure
I just need two stages because I know that stage1 C miscompiles stage2
Fortran. I use all-stage2, rather than bootstrap2, to avoid building
target libraries. This is like doing "make -C gcc bootstrap2" in the
old scheme.
make all-stage2 STAGE1_LANGUAGES=c,fortran
I use the stage1 compiler to make the target libraries, to avoid that
the miscompilation hits in libgfortran.
make stage1-start
make all-target-libgfortran
Unfortunately, all target directories are staged, even non-bootstrapped
ones, because of multilibbing. So I copy everything in the stage1
target directory to the stage2 target directory. But this annoyance is
actually good because in the future it will allow me to use the same
libgcc for both executables. This shields from potential
miscompilations of the stage2 C front-end (we have a buggy stage1
compiler!).
make stage # This is just for simplicity
cp -R stage1-powerpc*/* stage2-powerpc*/
Run stage2's regression tests (in this case I could have found the
failing testcase from the PR, but I was disconnected from Internet):
make stage2-start ; make -C gcc check-fortran
As soon as a test fails, make sure it did not fail in stage1.
make stage1-start ; make -C gcc check-fortran
RUNTESTFLAGS=dg.exp=complex_intrinsic_1.f90
Find out the command line to f951 (in reality, I cut & pasted from the
dejagnu log file and just added -### to the end).
gcc/gfortran -Bgcc
../gcc//testsuite/gfortran.dg/complex_intrinsic_1.f90 -S -###
Ok, we are ready to debug. In this case, I prefer to have two distinct
debuggers, and having directories
names stage1-gcc and stage2-gcc actually helps me. Hence the first make
invocation.
make stage
cp ../gcc/testsuite/gfortran.dg/complex_intrinsic_1.f90 aaa.f90
stage1-gcc/f951 aaa.f90 -fPIC -o stage1.s
stage2-gcc/f951 aaa.f90 -fPIC -o stage2.s
diff stage1.s stage2.s
This was just to be sure that I identified a failure mode. After
reducing the testcase a bit, I set to find the front-end miscompilation.
gdb --args stage1-gcc/f951 aaa.f90 -o stage1.s
gdb --args stage2-gcc/f951 aaa.f90 -o stage2.s
... some time later a.c contains the reduced testcase for the Fortran
front-end miscompilation, and I don't need anymore to work with two
stages at a time. Working in a gcc directory now will be like working
in a non-bootstrapped build.
make stage1-start
cd gcc
gdb --args ./cc1 -O2 -o - ../a.c
... fix bug in combine.c ...
make cc1
./cc1 -O2 -o - ../a.c
... seems fixed ...
./xgcc -B. -O2 -o test ../a.c
./test
Ok, looks like I'm done. Since stage1 miscompiled stage2, I expect many
if not all object files to change, and I cannot do a bubblestrap. There
is no equivalent of `make restrap', so I manually delete stage2.
cd ..
rm -rf stage2-* stage1-powerpc*
make bootstrap
make -k check
(It occurs now to me that restrap is easy to implement, so I will submit
a patch as soon as the queue is exhausted). A few hours later... yay,
bug fixed!
Paolo
